MIAMI - A tropical depression has begun strengthening in the Pacific off southwest Mexico and authorities have issued a tropical storm warning for a stretch of that country's coastline.

The U.S. National Hurricane Center in Miami said early Sunday the tropical depression is centred about 275 miles (445 kilometres) southwest of Acapulco, Mexico, and is moving northward at 10 mph (17 kph). The storm has top sustained winds of 35 mph (55 kph).

Mexico's government issued a tropical storm warning for its Pacific coast from Punta San Telmo to Manzanillo. The Miami centre says the system is strengthening and is expected to become a tropical storm later Sunday.

Forecasters say the system is expected to pass near the southwest coast of Mexico in the warning area by Monday.